1. Field of the Invention
The present invention relates to a process for preparing 1-chloro-1,2,2-trifluoroethylene or 1,2,2-trifluoroethylene in high selectivities and yields, which product is useful as a raw material for the preparation of polychlorotrifluoroethylene and substituted fluorohydrocarbons.
2. Description of the Related Art
Japanese Patent Publication No. 8454/1968 discloses a process for preparing 1,2,2-trifluoroethylene (hereinafter referred to as "3FH") by reacting 1,1,2-trichloro-1,2,2-trifluoroethane (hereinafter referred to as "R-113") and hydrogen at a temperature of 200.degree. to 300.degree. C. in the presence of a catalyst comprising palladium supported on activated carbon (Pd/carbon). By this process, 1-chloro-1,2,2-trifluoroethylene (hereinafter referred to as "3FCL") is obtained together with 3FH, but selectivities of other by-products are high and therefore the selectivity of 3FH and 3FCL is only 40 to 85%.
By a process of preparing a mixture of 3FH and 3FCL from R-113 and hydrogen in the presence of a platinum group metal supported on alkali magnesium fluoride as a catalyst which is disclosed in EP-A-063 657, 3FH and 3FCL there are obtained at a high selectivity of 90% or higher. But, the reaction temperature should be as high as 500.degree. C. By conducting the reaction at a low temperature of 200.degree. to 300.degree. C. in the presence of the above catalyst containing 0.5% of palladium, the selectivity remains low.
For the preparation of 3FCL, various other processes are also known. One example is a non-catalytic process comprising dechlorination of R-113 with zinc in a liquid phase (cf. Japanese Patent Publication Nos. 45322/1972, 5207/1982 and 5208/1982), and another is a catalytic process comprising dechlorination of R-113 with hydrogen in a gas phase (cf. Japanese Patent Publication No. 26484/1972, Japanese Patent Kokai Publication Nos. 185734/1985, 61936/1987 and 29328/1989 and GB Patent No. 698,386).
By using the liquid phase non-catalytic process with zinc, a comparatively high yield is obtained, but the production of by-produced zinc chloride is troublesome. In addition, the liquid phase process should use an anhydrous organic solvent. Using the gas phase dechlorination proceedure, the satisfactory high selectivity is achieved only under reduced pressure (for example, a selectivity of 95% under 310 Torr. in Japanese Patent Kokai Publication No. 61936/1987) but at a high reaction temperature (for example, a selectivity of 82 to 95% at 500.degree. C. in EP-A-053 657). The catalyst life is limited due to serious coke-poisoning.